I'm currently working on the handling to Jumping Gemini and I'm having trouble with one particular part.

I don't want to go too much into detail as I'm sure the effect is still marketed but the second revelation requires that you perform a DL and turnover while holding a card in your hand and I'm finding it problematic.
To clarify I'm having trouble performing a clean DL with a card in my hand.

Any tips or relevant literature anyone could point me to would be greafully accepted

Here's a guess - the diminishing lift sequence:
After you turn down the 'top' 10S and thumb it over and into your other hand, take it deeper into your fingers than your very fingertips. IE it gets gripped the joints of your second finger and thumb rather than the fingertips. This leaves your fingertips free to reciver the double pushoff and your thumbtip free to first grip then flipover what looks like a second 10s.

For the diminishing lift sequence, I just do this: after turning the triple FD, I just take the top "10S" and hold it between my right index finger and thumb. The remaining 3 fingers are used to turn the double FU, then face down. The second "10S" is taken under the first "10S" in the right hand between the right index finger and thumb. And so on.

(For a more detailed description, please see pg 72 of "Paper Engine" by Aaron Fisher)